In recent years, expectations are growing for widespread use of various electric cars with a view to solving environment and energy problems. As a key to practical application of the electric cars, vigorous development of lithium ion secondary batteries as an on-board power supply for driving a motor, for example, is under way. However, in order for the battery to gain wide acceptance as the on-board power supply, the battery needs to have higher performance and be less expensive. There is also the need for the single-charge travel distance of the electric car to approach that of gasoline engine cars, giving rise to the demand for higher energy batteries.
In order to increase the energy density of a battery, it is necessary to increase the amount of electricity stored per unit mass of the positive electrode and the negative electrode. As a positive electrode material (active material for the positive electrode) with potential to address such need, a so-called solid solution system positive electrode is being studied. Among others, a solid solution (so-called Li-rich layered positive electrode material) of electrochemically inactive layered Li2MnO3 and electrochemically active layered LiAO2 (A is a transition metal such as Co or Ni) is expected to provide a high-capacity positive electrode material capable of exhibiting a large electric capacity in excess of 200 mAh/g (see Patent Document 1 below).